Modern automotive vehicle engines commonly employ vaporized injected fuel for combustion. At start-up, when the engine is not fully warm, the injected fuel is commonly cold. Cold fuel is harder to vaporize than warm fuel. Consequently, some of the fuel may remain in a liquid state when injected. The injected liquid fuel tends to lead to decreased combustibility at start-up. This may result in undesirable emission levels.
To improve emission levels, different techniques have been employed before and after combustion. One pre-combustion treatment has been to heat the fuel prior to its injection. By heating the fuel, it becomes more easily vaporized thereby improving its combustibility. While successful, such pre-combustion heating is complex and expensive to implement. A common post-combustion treatment involves the employment of a catalyst in the engine exhaust gas stream. The catalyst burns the undesirable exhaust gas constituents prior to their passage to the atmosphere.
Modern automotive vehicles are also commonly equipped with a fuel vapour purge control system. Such a system accommodates fuel within the fuel tank which tends to evaporate as temperatures increase. The evaporated fuel collects in the fuel tank and is periodically removed by the purge vapour control system. The fuel vapours from the tank are initially collected and stored in a vapour canister. When the engine operating conditions are suitable for purging, a purge valve is opened permitting the engine to draw the fuel vapours from the purge canister into the engine for combustion.
A known system is disclosed in U.S. Pat. No. 6,234,153, describing a purge assisted fuel injection system and a method of using the same. The system includes a fuel tank coupled to a purge vapour collection canister by a vapour line. The purge vapour collection canister is coupled to a fuel injector operatively associated with an internal combustion engine by a second vapour line. A purge vapour canister vent valve selectively seals the purge vapour canister from atmosphere such that the fuel tank, purge vapour canister, and fuel injectors may form a closed system.
A vehicle provided with such a control system allows purge fuel vapour from the canister and liquid fuel to be selectively supplied to the engine via combined fuel and vapour injectors at start-up. However, such a system does not take into account that, during start-up, fuel vapour may be present in the liquid fuel supply line, commonly termed fuel rail, supplying the fuel injectors. One problem is that the fuel injectors are designed and controlled to deliver fuel in the liquid state. Hence they cannot be accurately controlled to deliver fuel in the vapour state, in order to achieve a desired Air/Fuel ratio in the engine combustion chambers.
At engine start the fuel injectors are controlled to open and deliver an appropriate amount of fuel to start and run the engine. The amount of fuel delivered is programmed in an engine management system computer as a function of coolant (water or oil) temperature, air temperature and other parameters. Prior to delivery the fuel is contained in a fuel rail upstream of the injectors which is held under pressure.
The above described strategy is dependent on the fuel in the fuel rail and in the injectors being in the liquid state. A further problem is that the fuel in the fuel rail can evaporate at higher fuel rail and injector temperatures, resulting in fuel vapours, or a mixture of fuel vapours and liquid fuel, being delivered when the fuel injectors open during an engine crank. This may occur, for instance, if an engine is re-started while still warm. The tendency for the fuel to evaporate will increase with increasing fuel rail temperature, decreasing fuel rail pressure and increasing fuel volatility. Residence time of the fuel in the fuel rail, that is, the time that has passed since the engine was last operated, may also affect the fuel evaporation. However, once the engine has been started the pressure in the fuel line will increase and the temperature will be lowered by the relatively cold fuel pumped from the fuel tank. After a period of time, liquid fuel will again be available for supply to the liquid fuel injectors.
Starting and operating the engine with evaporated fuel in the fuel rail may result in a too lean Air/Fuel ratio during and after start, causing rough engine running, misfire and poor engine performance. In severe cases it can lead to the engine failing to start.
The tendency for the fuel to evaporate in the fuel rail is a function of the fuel rail pressure, fuel volatility and fuel rail temperature. If the fuel rail pressure is increased during engine off periods this tendency of fuel evaporation is decreased, However, a further problem is that by increasing the fuel rail pressure before the engine is switched off, the evaporate emissions from the fuel system may increase due to increased leakage from the fuel through the injectors and other fuel system couplings and connections.
The present invention is therefore directed to solve the above problems by providing an improved injection control system for supplying fuel to an internal combustion engine during start-up.